If OP inherits the house where DM lives, with DM having a lifetime interest in the property (ie she can live there until she dies), then DM can't sell it to pay for care fees because DM doesn't own it.
If DM doesn't have a lifetime interest in it then she's simply living in a house owned by someone else and is at the mercy of being thrown out by OP if OP wanted or needed to sell the house to pay for inheritance tax.
This is why people don't give their property to their DC whilst the parents are still alive, because it puts them at the mercy of the DC as to whether or not they're made homeless.
The issue for you OP is one of opinions and whether members of your family think everything should be equal shares inheritance. Although it's really only the opinion of the person leaving the inheritance that matters, legally. As it stands, it's not "fair" as in an equal split with you and DSis.
DSis gets cheap rent for years and then inherits a house, which she plans to live in. Ok, but how is she going to pay the inheritance tax on the value of that house if she's presumably poor? She'd also get the disregarded part of the house's value under inheritance tax rules (your DF inheritance tax allowance, which that allowance would be split between you and her).
Meanwhile, you inherit a house you can't sell because your DM has a lifetime interest in it. You have to pay inheritance tax on the part that isn't disregarded, (your DF inheritance tax allowance, with that allowance being split between you and DSis). You're still responsible for maintaining the house though, even though you don't live there. Fine if you're rich, not so great if you're struggling to even pay your own house (that you live in) repairs, assuming you already own. If you are currently renting, it's going to suck a bit knowing you have a house which you own, but can't live in it until DM dies so you have to keep paying rent to some landlord.
If DM inherits the house she lives in and not you, then 1) DSis gets DF entire inheritance tax allowance disregard all to herself. So less inheritance tax to pay for her. 2) DM house which she now inherited, it'll be sold for care fees (if she needs care), and has insufficient other funds to pay for it (a care home is thousands per week). Then you'd inherit nothing. Or 3) If she doesn't need care and dies still owning the house, then you inherit it and have the entire disregarded initial part due to DM inherence tax allowance, so less inheritance tax to pay for you.
So DSis gets a house regardless, which she may have to sell to pay some inheritance tax. She buys something she can afford and carries on with her life.
But your situation is more complicated because yours isn't a straight inheritance. There's financial implications for you. You may, depending on how it's set up, inherit nothing if house is inherited first by DM and ends up sold to pay for DM care. Or you may inherit a house you are legally obligated to carry out repairs on at a time in life you maybe can't afford those repairs, but you can't sell the house instead. You could also be liable for inheritance tax but unable to legally sell the house to raise the money to pay it.
If your DF wants to make it equal he should leave it so that both of you jointly inherit both houses. That way whatever the financial implications are, both you and DSis meet them equally. Your DSis would have to buy you out of your half of the house she lives in, if she wanted to continue living there and both of you would have to raise money elsewhere to pay the inheritance tax if you're not raising that money by selling the house DSis lives in. This assumes there's no other assets eg cash to be inherited which could be used to pay inheritance tax.
Basically DF is trying to prevent the house where DM lives being sold for care fees by leaving it to you, not DM, so he's trying to give you the chance to inherit the house, but the risk is there may be financial implications for you. He's taking a gamble on which is most likely, DM needing care in old age (so then best option is to leave house to you) or you being too poor to afford inheritance tax/house repairs on a house you can neither sell nor live in (so then best option is to leave house to DM and DM leaves it to you if she still has it at time of her death).
Sorry if I've made a hash of explaining it!